1. Field of the Invention
The present invention relates to video coupling circuits and, more specifically, to video coupling circuits for use in radar applications.
2. Description of the Prior Art
The sensitivity of radar receivers is ordinarily limited by the noise produced within the receiver. However, extraneous radiation may be picked up on the radar antenna, and various antijamming provisions are included in most radar receivers to overcome this type of interference. Additionally, a filter is often inserted between the detector and video amplifier that will pass individual video pulses but discriminate against frequencies associated with spurious echoes such as those caused by ground, sea or rain clutter. Several types of filter have been tried, but the simplest is the fast time constant (FTC) circuit, which is basically an R-C differentiating network inserted in the input circuit of the video amplifier. Such video coupling circuits must meet strict requirements if proper transient behavior is to be realized. If high frequency response is unduly limited, serious lengthening of the pulse signals may occur. If imperfect low frequency response is encountered, serious overshoot may occur in the voltage pulses. FTC circuits not only furnish some protection against interference, but also increase the contrast between single radar echoes in the presence of extraneous rain clutter signals.
In general, FTC circuits employ an impedance-coupling network in which the time constant can be varied to optomize the contrast between desired signal and clutter.
Some FTC circuits provide means for selecting between discrete values of time constant in the coupling network. In such circuits, the value of the FTC time constant is changed by means of a remote FTC switch. However, circuits of this type are limited by the number of switch positions that can be incorporated economically into the equipment.
Still other prior art FTC circuits provide continuously variable time constants by including a varactor diode as a variable capacitor or a diode as a variable resistor. However, circuits of this type suffer from a lack of dynamic range in the time constant adjustment.